Ok thanks @tkhunny @staple @Denis and @JeffM

So 80% of 60 = 48 (0.8 * 60)
And 60% of 30 = 18 (0.6 * 30)

its easy to work out that bit, although I do it the long way round I just say whatís 10% of 60 = 6 then x that 6 x 8 = 48
And 10% of 30 = 3 then times that by 6 = 18

And to check my work I type in Bing 60% of 30 and it tells me

60% of 30 = 18

Not pretty but itís the only way I can do it in my head 😂

Then for working out the percentages of the Apple and Blackcurrant mix, Iím not too sure on the calculation, although it would be handy to know you guys have shown me that the answer is kind of already there.

I usually just use Bing and type 18 as a % of 30

and is says

18 is 60% of 30 😀

So now Iím wondering how Bing works out X as a percentage of Y and how would all of these calculations go into one big sum, with brackets and stuff so that if you had an imaginary calculator that had a drop down box to choose the ratio for the final concentrate at either 80/20 70/30 or 60/40 and a box to input how much Apple was in the vial already. Then a calculate button, which would show exactly what amount of Apple and Blackcurrant in ml would need to be added to the extra mix to make the total mix whichever ratio was chosen in the drop down box and also display the ratio in percent for the additional mix. That would be cool.

So in the imaginary calculator if 80/20 were chosen from the drop down field and 30ml was input into the field for how much Apple was already in the vial (using the last example), behind the scenes the imaginary calculator would go...

30 - (0.8 * 60 - 30)
Which would give (18ml Apple) and 12ml Blackcurrant
Then hopefully the calculator would have an internet connection and ask Bing
100 - (18 as a percentage of 30)
which would give 60/40